Makes sure the media modal dialog `h1` heading isn't empty when custom media frames don't set a title. This is particularly important now that the media modal is an ARIA dialog, as the title is referenced by an `aria-labelledby` attribute to properly label the dialog.
Props donmhico, audrasjb.
Fixes #47612.
git-svn-id: https://develop.svn.wordpress.org/trunk@45925 602fd350-edb4-49c9-b593-d223f7449a82